1. Field of the Invention
The present invention generally relates to protecting composite structures and assemblies from lightning strikes, and more particularly, to protecting composite aircraft from arcing and internal sparking from lightning strikes.
2. Background Description
Increasingly, aircraft manufacturers are replacing aluminum in aircraft with carbon fiber composites that generally have a higher strength-to-weight ratio than aluminum. While, aluminum and other metals are highly conductive, a typical state of the art composite, such as Carbon Fiber Reinforced Plastic (CFRP), is much more resistive. Composite components are typically bolted to one another or to other non-composite (e.g., metal) components, e.g. a composites skin may be fastened or bolted to a frame (metal or composite).
A transient charge such as from a lightning strike to a body discharges into the body. So, when an airplane is struck by lightning, typically, the charge disperse as current flowing both through the skin and through the substructure. In a metal body the current from the discharge distributes relatively evenly over the metal body. In a composite structure the resistive composite surface may be attached by low resistance conductive metal fasteners that may or may not be attached to internal metal or sub-structure, e.g., structural spars for wings, fuselages, fuel tanks, and other components. The same charge disperses over an electrically irregular composite surface (i.e., large stretches of higher resistance composite surface interrupted by low resistance surface features such as fasteners) and discharge un-evenly.
So typically, in a composite structure current passes laterally from the fasteners through adjacent composite skin and enters the substructure through the fasteners and from the fasteners into sub-structure. A lightning strike in the immediate vicinity of a fastener, or actually to a fastener, produces a very high current density though the fastener. A fastener passing through a fastener hole in the skin may not contact the skin directly. The potential difference between the fastener and the fastener hole sidewall can cause sparks (e.g., from arcing) and hot gases in the hole that may eject from the fastener. Consequently, composite aircraft require additional and often costly lightning protection features, e.g., on the composite wing surface to manage the current path for such discharges and for preventing sparks and hot gases from entering the structure.
Spot facing is one approach to improving the pressure-containing seal for the hole. Spot bonding may be used in conjunction with spot facing and involves removing coatings from fastening hardware and related structure to ensure good electrical contact. However, spot facing is time consuming to apply to each location or even to selected locations. Spot bonding adds even more time. Other typical state of the art lightning protection approaches focus on diverting the resultant current and/or isolating conductive features from the current. Unfortunately, adding these lightning protection features negatively impact manufacturing cycle time, e.g., the time required to produce a wing or to fix the skin to the sub-structure.
Thus, there is a need to reduce or eliminate internal sparking and prevent hot gas from escaping into composite structures and especially, there is a need for a low cost easy to implement method for reducing and/or eliminating internal sparking and hot gasses within the fueled areas in composite aircraft.